From b3fd6ddf73aeefd9d3bb2e92f463ce20be35bb8e Mon Sep 17 00:00:00 2001
From: emaste <emaste@FreeBSD.org>
Date: Wed, 21 Feb 2018 15:54:23 +0000
Subject: [PATCH] ministat: disallow negative variance / nan Stddev

With all values identical it was possible for Var() to return a negative
value due to limited floating point precision, resulting in "nan"
reported as Stddev.

Variance cannot actually be negative, so just return 0.  We can later
investigate alternate algorithms for calculating variance to reduce the
effect of catastrophic cancellation here.

Reported by:	Arshan Khanifar <arshankhanifar_gmail.com>
Approved by:	phk
Sponsored by:	The FreeBSD Foundation
---
 usr.bin/ministat/ministat.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/usr.bin/ministat/ministat.c b/usr.bin/ministat/ministat.c
index 625fb329e2b8..150593e5be95 100644
--- a/usr.bin/ministat/ministat.c
+++ b/usr.bin/ministat/ministat.c
@@ -208,6 +208,12 @@ static double
 Var(struct dataset *ds)
 {
 
+	/*
+	 * Due to limited precision it is possible that sy^2/n > syy,
+	 * but variance cannot actually be negative.
+	 */
+	if (ds->syy <= ds->sy * ds->sy / ds->n)
+		return (0);
 	return (ds->syy - ds->sy * ds->sy / ds->n) / (ds->n - 1.0);
 }